1. Field of the Invention
This invention relates to a mechanism for holding a focusing lens in an optical system (hereinafter referred to as "a focusing lens holding mechanism" or "a holding mechanism", when applicable), and more particularly to a focusing lens holding mechanism in an image forming apparatus such as a copying machine which is adapted to form the image of an original as a latent image on the surface of a photo-sensitive member.
2. Description of Conventional Art
In general, an image forming apparatus operates as follows: The image of an original focused by the optical system is applied to a photo-sensitive member, so that its electrostatic latent image is formed thereon. The latent image is developed into a toner image by the developing unit, and the toner image is transferred onto a printing paper. There have been proposed a variety of image forming apparatuses in the art (cf. Examined Japanese Patent Publication (Kokoku) Hei-2-17453/(1990).
The optical system in the image forming apparatus comprises: a light source for irradiating an original; and a focusing lens for leading light reflected from the original thus irradiated to a photo-sensitive member so as to form the image of the original thereon.
In installation of the focusing lens, it is essential to perform the following three adjusting operations: In the first adjusting operation, in order to adjust the focal length of the focusing lens, the length of the optical path is adjusted. In the second adjusting operation, in order to eliminate the inclination of the image focused on the photo-sensitive member by the focusing lens, the resolving-power balance is adjusted. In the third adjusting operation, in order to align the optical axis of the focusing lens with the paper conveying reference axis of a paper conveying section, the optical axis is adjusted.
There has been available a focusing lens holding mechanism with which the above-described adjusting operations can be achieved. The holding mechanism is designed as shown in FIG. 3. That is, a focusing lens 31 is held with a holder 32. The holder 32 has a mounting part, which is mounted on the lower surface of a base member 33 set over a photo-sensitive drum (not shown). The mounting part of the holder 32 has a pair of screw holes j at both ends, respectively. More specifically, the screw holes j are positioned on both sides of the optical axis of the focusing lens 31, respectively. Screws m are threadably engaged with the screw holes j from above the base member 33, so that the focusing lens 31 is fixedly mounted on the base member 33 through the holder 32. In order to threadably engage the screws m with the screw holes j, a pair of slots n are formed in the base member 33 in such a manner that they are in parallel with each other and the optical axis P1 of an optical system (not shown) is at the middle of the distance between them.
With the mechanism thus constructed, the length of the optical path of the focusing lens 31 is adjusted by changing the position of the holder 32 in the range defined by the length of the slots n. Furthermore, the resolving power balance and the optical axis are adjusted by changing the position of the holder 32 in the range defined by the clearances between the slots n and the screws m.
As was described above, in the case of the conventional mechanism, the resolving power balance and the optical axis are adjusted by changing the position of the holder 32 in the range defined by the clearances between the elongated holes n and the screws m. Hence, it is impossible to perform the adjusting operations separately, and therefore it is essential to manufacture the components with severe tolerances (for instance .+-.0.05 to 0.40 mm). Therefore, the components are low in yield rate when manufactured, and accordingly high in manufacturing cost.
Furthermore, the above-described holding mechanism may be applied to a so-called "clamshell type image forming apparatus" in which the upper unit is coupled to the lower unit in such a manner that the former is swingable with respect to the latter. In this case, the optical system is mounted, as an additional unit, on the photo-sensitive member. Therefore, if the coupling of the upper and lower units is low in accuracy, then after the optical system has been installed, its resolving power balance may be changed, or the optical axis may be shifted. In this case, the following difficulties will occur because, with the conventional holding mechanism, the adjusting operations cannot be performed separately. That is, even in the case where it is required to adjust the resolving power balance only or the optical axis only, it is necessary to disassemble the optical system and reassemble it while adjusting the resolving power balance and the optical axis again. Thus, once the optical system is installed, it is rather difficult to perform the adjusting operations for it again.